1. Field of the Invention
The present invention relates to a control system using a gas sensor element for various control operations, such as controlling the introduction of outside air into the passenger compartment of an automobile by detecting a variation in the concentration of a gas in the environment. More particularly, the invention relates to a control system using a gas sensor element capable of lessening the effect of variations in sensor resistance among gas sensor elements as well as the effect of variation in the sensor resistance of the gas sensor element caused by environmental factors, such as temperature and humidity.
2. Description of the Related Art
Since the sensor resistance of a gas sensor element using a WO3 thin film, lead-phthalocyanine, or SnO2 varies with the concentration of a specific gas, such as NOx, CO, or HC (hydrocarbon) contained in the environment, a conventionally known gas sensor element detects a variation in the concentration of a specific gas as a function of sensor resistance. For example, by using such a gas sensor element, a known control system opens/closes a flap for introducing outside air into the passenger compartment of an automobile according to the condition of contamination of outside air, or controls an air cleaner upon detection of contamination of air within the passenger compartment due to smoking.
In many cases, a control system using such a gas sensor element detects variation in sensor resistance in the form of an electric signal in the following manner. A gas sensor element having a sensor resistance Rs, and a detection resistor having a predetermined detection resistance Rd are connected in series. A predetermined direct-current voltage is applied between opposite ends to thereby divide the voltage by means of the gas sensor element and the detection resistor. On the basis of a divided voltage appearing from a point between the gas sensor element and the detection resistor, various processes are performed.
However, the sensor resistance Rs of a gas sensor element may be greatly influenced by factors, such as temperature and humidity, of the environment in which the gas sensor element is placed, as well as by the concentration of a specific gas, such as NOx, to be detected. Due to environmental factors, such as temperature and humidity, the sensor resistance Rs of the gas sensor element and the detection resistance Rd of a detection resistor may differ greatly in an arrangement where the concentration of a specific gas is determined by detecting a variation in the potential obtained by applying a predetermined voltage to a voltage divider comprising the gas sensor element and the detection resistor as described above. As a result, the potential obtained by the voltage divider may be biased near the predetermined potential or ground potential. Thus, the sensor resistance Rs; i.e., variation in the concentration of a specific gas, cannot be detected accurately.
Also, since sensor properties are not completely uniform among gas sensor elements, even when similar gas sensor elements are used, the sensor resistance Rs; i.e., an output, may vary among the gas sensor elements.
An object of the invention is to provide a control system using a gas sensor element capable of lessening the effect of variations in sensor properties among gas sensor elements as well as the effect of environmental factors, such as temperature and humidity, to thereby accurately detect variation in the concentration of a specific gas.
To achieve the above object, the present invention provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor via a charge resistor during a period when the pulse signal in the first potential state is input to the pulse input point; and a discharging circuit for discharging the capacitor via a discharge resistor during a period when the pulse signal in the second potential state is input to the pulse input point. The gas sensor element having a sensor resistance comprises at least either the charge resistor of the charging circuit or the discharge resistor of the discharging circuit. Furthermore, at least either the charging current of the charging circuit or discharging current of the discharging circuit varies with the sensor resistance of the gas sensor element. The control system further comprises a control circuit, which in turn comprises a microcomputer; and an A/D converter circuit for converting a potential at an operating point located at one end of the capacitor to a digital valve, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged and discharged by means of the pulse signal. Also, at least either the charging current during charging or discharging current during discharge varies with the sensor resistance of the gas sensor element. The voltage across the capacitor becomes steady by repeated charging and discharging according to the pulse signal, and the charging voltage of the capacitor (a voltage as measured across the capacitor) varies with the sensor resistance. Thus, when the sensor resistance of the gas sensor element varies as a result of the gas sensor element detecting the specific gas, the charging voltage of the capacitor varies accordingly. As a result, the potential of the operating point located at one end of the capacitor varies accordingly. Thus, an A/D-converted value of the potential varies according to the concentration of the specific gas. Therefore, variation in the concentration of the specific gas can be known from the A/D-converted value.
In the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude (the difference between the first potential and the second potential) of the pulse signal, which is output from the control circuit and is input to the pulse input point. Accordingly, when the sensor resistance of the gas sensor element varies due to variation in environmental factors, such as temperature or humidity, the duty ratio, for example, of the pulse signal is varied appropriately to prevent large biasing of the potential at the operating point or an A/D-converted value of the potential, thereby maintaining the value within an appropriate range. Thus, even in this case, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties, such as sensor resistance (for example, a sensor resistance as obtained in an environment of a standard gas concentration at predetermined temperature and humidity), vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point (an A/D-converted value of the potential), thereby maintaining the value within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
The pulse signal is not particularly limited, so long as the pulse signal has a waveform which alternates between a first potential state and a second potential state. The first potential and the second potential may be set for use with a single-polarity power source; for example, either the first potential or the second potential assumes +5V, whereas the other assumes 0 V (ground). Alternatively, the first potential and the second potential may be set for use with a dual-polarity power source; for example, either the first potential or the second potential assumes +5V, whereas the other assumes xe2x88x925 V. In order to vary the potential at the operating point, PMW (pulse width modulation) or amplitude modulation for varying the duty ratio of the pulse signal is employed.
The charging circuit is not particularly limited, so long as the charging circuit can charge the capacitor according to the pulse signal in the first potential state which is input to the pulse input point. The discharging circuit is not particularly limited, so long as the discharging circuit can discharge the capacitor according to the pulse signal in the second potential state which is input to the pulse input point.
However, the charging circuit and the discharging circuit must be configured such that at least either the charging current of the charging circuit or the discharging current of the discharging circuit varies with the sensor resistance of the gas sensor element. For example, the charging circuit may assume the form of a CR series circuit composed of a charge resistor having a resistance Rc and a capacitor having a capacitance C, which are connected in series, and having a first time constant xcfx841 of CRc. The discharging circuit may assume the form of a CR series circuit composed of a discharge resistor having a resistance Rd and a capacitor having a capacitance C, which are connected in series, and having a second time constant xcfx842 of CRd. Alternatively, an active element, such as a transistor, an FET, or an operational amplifier, may be used to cause current to flow into or out of the capacitor according to the sensor resistance of the gas sensor element or the resistance of a resistor, to thereby charge or discharge the capacitor.
The charging circuit may be configured such that charging current flows into the capacitor through the pulse input point or such that charging current flows into the capacitor from a separate power source via a switching element, which is driven by an input pulse signal.
The discharging circuit may be configured such that discharging current flows from the capacitor toward the pulse input point or such that the capacitor discharges via a switching element, which is driven by an input pulse signal.
The A/D converter circuit may be configured so as to directly A/D convert the potential at one end of the capacitor or such that a buffer circuit is disposed at a stage preceding A/D conversion. Alternatively, a low-pass filter (LPF) having a cut-off frequency lower than a frequency fp of the pulse signal to be input, or a band elimination filter (BEF) for cutting off signals having frequencies in the vicinity of the frequency of the pulse signal may be disposed at a stage preceding A/D conversion.
In particular, by sufficiently shortening the repetition period Tp of the pulse signal; i.e., by increasing the frequency fp, rippling decreases, whereby the potential at one end of the capacitor (at the operating point) becomes substantially constant. Thus, even when the potential at the operating point is directly A/D-converted, the A/D-converted value is not susceptible to fluctuations associated with charging and discharging. Therefore, an LPF or a like component becomes unnecessary.
That is, preferably, the control system is configured in the following manner. The frequency of the pulse signal is determined such that, within the range of variations in the sensor resistance of the gas sensor element, the maximum ripple value arising in the potential at the operating point becomes smaller than the resolution of the A/D converter circuit. The A/D converter circuit directly converts the potential at the operating point to a digital signal.
A resistor other than the gas sensor element which can be used as the charge resistor or the discharge resistor is a fixed resistor, which has a constant resistance. Alternatively, a variable resistor can be used. The variable resistor can accommodate a sensor resistance Rs which varies over a range of several orders of magnitude, by appropriately modifying its resistance. Variations in properties among gas sensor elements can be compensated by adjusting the resistance of the variable resistor. Further alternatively, another gas sensor element having different properties; for example, a gas sensor element whose resistance varies in response to a gas different from the above-mentioned specific gas, can be used as the charge resistor or the discharge resistor.
Preferably, a control system using a gas sensor element whose sensor resistance varies with the concentration of a specific gas comprises a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a charge resistor during a period when the pulse signal in the first potential state is input to the pulse input point; and a discharging circuit for discharging the capacitor at a second time constant via a discharge resistor during a period when the pulse signal in the second potential state is input to the pulse input point. The gas sensor element having the sensor resistance comprises at least either the charge resistor of the charging circuit or the discharge resistor of the discharging circuit, and at least either the first time constant or the second time constant varies with the sensor resistance of the gas sensor element. The control system further comprises a control circuit, which in turn comprises a microcomputer; and an A/D converter circuit for converting a potential at an operating point located at one end of the capacitor to a digital signal, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point and outputs the pulse signal.
The control system for a gas sensor element of the above embodiment is similar to that described previously, but is characterized in that the capacitor is charged or discharged by means of the pulse signal and that at least either the first time constant, which is a time constant for charging, or the second time constant, which is a time constant for discharging, varies with the sensor resistance of the gas sensor element. Accordingly, by repeatedly charging and discharging according to the pulse signal, the voltage across the capacitor becomes steady, and the charging voltage of the capacitor varies with the sensor resistance. Thus, when the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
The charging circuit is not particularly limited, so long as the charging circuit can charge the capacitor according to the pulse signal in the first potential state which is input to the pulse input point. The discharging circuit is not particularly limited, so long as the discharging circuit can discharge the capacitor according to the pulse signal in the second potential state which is input to the pulse input point. However, the charging circuit and the discharging circuit must be configured such that at least either the first time constant of the charging circuit or the second time constant of the discharging circuit varies with the sensor resistance of the gas sensor element.
Although the A/D converter circuit may be configured so as to directly convert the potential at the operating point to a digital value, a buffer circuit may be disposed at a stage preceding to A/D conversion, and an LPF having a cut-off frequency lower than a frequency fp of the pulse signal or a BEF for cutting off frequencies in the vicinity of the frequency of the pulse signal may be interposed.
In particular, by sufficiently shortening the repetition period Tp of the pulse signal as compared with the first time constant xcfx841 and the second time constant xcfx842 (Tp less than  less than xcfx841; Tp less than  less than xcfx842), a potential at one end of the capacitor (at the operating point) becomes substantially constant. Thus, even when the potential at one end of the capacitor is directly A/D-converted, the A/D-converted value becomes unsusceptible to fluctuations associated with charging and discharging. Therefore, an LPF or a like component, becomes unnecessary. That is, preferably, the control system is configured in the following manner. The repetition period Tp of the pulse signal assumes a value sufficiently smaller than the first time constant xcfx841 and the second time constant xcfx842. The A/D converter circuit directly converts the potential at the operating point to a digital value.
The present invention further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a charge resistor during a period when the pulse signal in the first potential state is input to the pulse input point; a discharging circuit for discharging the capacitor at a second time constant via a discharge resistor during a period when the pulse signal in the second potential state is input to the pulse input point, the gas sensor element having the sensor resistance comprises the discharge resistor, and the second time constant varying with the sensor resistance; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point and outputs the pulse signal.
In the control system for a gas sensor element of the above embodiment, the capacitor is charged or discharged by means of the pulse signal, and the second time constant, which is a time constant for discharging, varies with the sensor resistance of the gas sensor element. Accordingly, by repeatedly charging and discharging according to the pulse signal, the voltage across the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. Thus, when the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal, which is input to the pulse input point from the control circuit. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined.
The present invention still further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a charge resistor during a period when the pulse signal in the first potential state is input to the pulse input point, the gas sensor element having the sensor resistance comprising the charge resistor, and the first time constant varying with the sensor resistance; a discharging circuit for discharging the capacitor at a second time constant via a discharge resistor during a period when the pulse signal in the second potential state is input to the pulse input point; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged or discharged by means of the pulse signal, and the first time constant, which is a time constant of charging, varies with the sensor resistance of the gas sensor element. Accordingly, by repeatedly charging and discharging according to the pulse signal, the voltage across of the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. Thus, when the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal, which is input to the pulse input point from the control circuit. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point (an A/D-converted value of the potential), thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined.
The present invention still further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a resistor and a diode during a period when the pulse signal in the first potential state is input to the pulse input point; a discharging circuit for discharging the capacitor at a second time constant via the gas sensor element during a period when the pulse signal in the second potential state is input to the pulse input point; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point of the charging circuit and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged via the resistor and the diode during a period when the pulse signal in the first potential state is input; and the capacitor is discharged via the gas sensor element; i.e., via the sensor resistance which varies with the concentration of a gas, during a period when the pulse signal in the second potential state is input. Accordingly, the first time constant for charging is determined by the resistance of the resistor, whereas the second time constant for discharging varies with the sensor resistance. By repeatedly charging and discharging according to the pulse signal, the voltage across of the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. Thus, when the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
The present invention still further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via the gas sensor element and a diode during a period when the pulse signal in the first potential state is input to the pulse input point; a discharging circuit for discharging the capacitor at a second time constant via a resistor during a period when the pulse signal in the second potential state is input to the pulse input point; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point of the charging circuit and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged via the gas sensor element and the diode; i.e., via the sensor resistance which varies with the concentration of a gas, during a period when the pulse signal in the first potential state is input; and the capacitor is discharged via the resistor during a period when the pulse signal in the second potential state is input. Accordingly, the second time constant for discharging is determined by the resistance of the resistor, whereas the first time constant for charging varies with the sensor resistance. Thus, by repeatedly charging and discharging according to the pulse signal, the voltage across the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. When the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
The present invention still further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a RD series circuit comprising a resistor and a first diode, which is connected to the resistor in series, during a period when the pulse signal in the first potential state is input to the pulse input point; a discharging circuit for discharging the capacitor at a second time constant via a SD series circuit comprising the gas sensor element and a second diode, which is connected to the gas sensor element in series, and connected to the RD series circuit in parallel, during a period when the pulse signal in the second potential state is input to the pulse input point; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point of the charging circuit and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged via the RD series circuit comprising the resistor and the first diode during a period when the pulse signal in the first potential state is input. The capacitor is discharged via the SD series circuit comprising the gas sensor element and the second diode; i.e., via the sensor resistance which varies with the concentration of a gas, during a period when the pulse signal in the second potential state is input. Accordingly, the first time constant for charging is determined by the resistance of the resistor, whereas the second time constant for discharging varies with the sensor resistance. Thus, by repeatedly charging and discharging according to the pulse signal, the voltage across the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. When the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
The present invention still further provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor at a first time constant via a SD series circuit comprising the gas sensor element and a first diode, which is connected to the gas sensor element in series, during a period when the pulse signal in the first potential state is input to the pulse input point; a discharging circuit for causing the capacitor at a second time constant via a RD series circuit comprising a resistor and a second diode, which is connected to the resistor in series, and connected to the SD series circuit in parallel, during a period when the pulse signal in the second potential state is input to the pulse input point; and a control circuit. The control circuit comprises a microcomputer; and an A/D converter circuit into which is input a potential at an operating point located at one end of the capacitor, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point of the charging circuit and outputs the pulse signal.
In the control system using a gas sensor element of the present invention, the capacitor is charged via the SD series circuit comprising the gas sensor element and the first diode; i.e., via the sensor resistance which varies with the concentration of a gas, during a period when the pulse signal in the first potential state is input. The capacitor is discharged via the RD series circuit comprising the resistor and the second diode during a period when the pulse signal in the second potential state is input. Accordingly, the second time constant for discharging is determined by the resistance of the resistor, whereas the first time constant for charging varies with the sensor resistance. Thus, by repeatedly charging and discharging according to the pulse signal, the voltage across the capacitor becomes steady. Furthermore, the charging voltage of the capacitor varies with the sensor resistance. When the gas sensor element detects the specific gas with a resultant variation in the sensor resistance thereof, the charging voltage of the capacitor varies accordingly; as a result, the potential at the operating point located at one end of the capacitor varies accordingly. Therefore, an A/D-converted value of the potential varies according to the concentration of the specific gas. Hence, variation in the concentration of the specific gas can be known from the A/D-converted value.
Furthermore, in the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude of the pulse signal. Accordingly, when the sensor resistance of the gas sensor element varies due to environmental factors, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent biasing of the potential at the operating point, thereby maintaining the potential within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
Preferably, in either of the above-mentioned two control systems, the second potential state is lower in potential than the first potential state, and the pulse input point is connected directly to the RD series circuit and the SD series circuit.
As a result, system configuration becomes simple, to thereby provide an inexpensive control system.
Preferably, in any one of the above-mentioned control systems, either the first potential state or the second potential state is a ground potential state, and the other state is a positive potential state, which is higher in potential than the ground potential.
One potential state of the pulse signal, which is output from the control circuit and input to the pulse input point, assumes the ground potential, and the other potential state assumes a potential higher than the ground potential. Thus, the control system can be powered by a single-polarity power source, thereby simplifying a power circuit for the system.
Preferably, in any one of the above-mentioned control systems, the control circuit further comprises: output range judgement means for judging whether or not an A/D-converted value produced in the A/D converter circuit falls outside a predetermined range; and duty ratio modification means for modifying the duty ratio of the pulse signal such that the A/D-converted value falls within the predetermined range (namely, when the A/D-converted value falls outside the predetermined range).
When the sensor resistance of a gas sensor element varies greatly due to variation in environmental factors, such as temperature or humidity, the A/D-converted value produced in the A/D converter circuit varies greatly. In this state, variation in the sensor resistance derived from variation in the concentration of the specific gas cannot be reliably detected.
By contrast, in the control system of the present invention, when the A/D-converted value falls outside a predetermined range, the duty ratio of the pulse signal is modified such that the A/D-converted value falls within the predetermined range. Accordingly, even when an environmental factor, such as temperature or humidity, varies, the A/D-converted value is maintained within the predetermined range without biasing. Thus, variation in the sensor resistance derived from variation in the concentration of the specific gas can be reliably detected.